This invention pertains to the field of electrical illuminating apparatus and particularly to such apparatus which is permissible under Mining Enforcement and Safety Administration (MESA) standards and regulations for use in explosive atmospheres such as coal mines.
Lighting in mines has always been relatively poor compared to working environments above ground where minimum illumination standards for various tasks have long been established.
The difficulty of providing adequate lighting in coal mines is aggravated by the low reflectivity of the black coal and associated minerals in the roof, floor, and side walls. Rock-dusting, where employed, provides a reflective white or light gray surface along established haulageways and heavy traffic areas such as underground maintenance shops, areas immediately adjacent the bottoms of hoisting shafts, and loading points along conveyors. These locations for the most part, are well illuminated with permanent lighting.
By contrast, rooms where coal is actively being mined are relatively poorly lighted. These rooms have not yet been rock-dusted and the freshly exposed black surfaces provide no practical reflectivity. Illumination is provided only by miners' cap lamps and one or more high intensity headlight-type lamps on each piece of mobile mining machinery. In the case of shuttle cars, which operate in both directions, there will be one or more headlights at each end. Even where such high intensity lamps are directed toward the face, or toward the direction of movement of the machine, lighting is far from uniform. The operator of a continuous mining machine, or loading machine, will have enough light brilliantly illuminating the mine face to keep the machine working efficiently, but the rear boom just behind the operator is in relative darkness making it difficult to see a person immediately behind or to the side. Inasmuch as these face-working machines have conveyor discharge booms which are tiltable up and down, and swingable from side to side, there have been numerous accidents involving persons unseen by the machine operators being struck by the discharge booms and pressed against another machine or one of the sidewalls.
MESA reports show that almost all serious and fatal accidents in working places occur while self-propelled equipment is operated in them.
Pursuant to authority under the Federal Coal Mine Health and Safety Act of 1969, the Secretary of the Interior has promulgated new illumination standards for underground coal mines which, among other things, specify that the entire area surrounding self-propelled mining equipment for a minimum distance of 5 feet be illuminated with a surface brightness of at least 0.06 footlamberts.
To provide this level of illumination, it is absolutely necessary that something more efficient than conventional incandescent lamps be used. As shown in applicant's above-identified co-pending patent applications, applicant has developed fluorescent lighting permissible for use in potentially explosive atmospheres such as coal mines, which can provide the high level illumination required by the new standards. However, even higher standards of illumination may be required in the future, and in more compact lamps.
High intensity discharge ("HID") lamps provide light levels and lumens per watt efficiencies greatly exceeding fluorescent lamps. For many years, virtually all of the HID lamps of commercial significance were mercury vapor lamps. They trace their history back to the Cooper-Hewitt lamp developed in 1901 by Peter Cooper Hewitt. Continuing research has developed the modern high pressure sodium lamp which is believed to be the most efficient lamp known among all light sources now commercially available. It provides up to 140 lumens per watt compared with 80 lumens per watt for fluorescent lamps, 50 lumens per watt for mercury vapor lamps, and 15 to 20 lumens per watt for incandescent lamps. The arc tube in the high pressure sodium lamp operates at such high temperatures (1300.degree. Celsius, 2,372.degree. Fahrenheit) that it must be made of something like translucent aluminum oxide. It would melt the high temperature glass, and even quartz, which are used in other types of lamps. This very high service temperature, and the extremely corrosive effects of hot sodium, have made it difficult to apply this lamp to use in underground mines having explosive atmospheres, even though the increased illumination would reduce accidents and personal injuries, and much of the electrical energy now used for lighting purposes would be conserved.